Description of the Related Art
In the past it has been common to provide a wide variety of molding systems including the molding of a thermoplastic resin or a thermoplastic composite part. In vacuum molding, a slab (constant thickness sheet) of heated thermoplastic material is placed on the vacuum mold and a vacuum drawn between the mold and the heated plastic material to draw the plastic material onto the mold. Similarly, in compression molding, a lump or slab of preheated material is pressed between two molding forms which compress the material into a desired part or shape.
Related Patents
Prior U.S. patents which use thermoforming of material can be seen in the four Winstead patents, U.S. Pat. Nos. 4,420,300; 4,421,712; 4,413,964; and 3,789,095. The Winstead '712 and '300 patents are for an apparatus for continuous thermoforming of sheet material including an extruder along with stretching means and a wheel having a female mode thereon and a plurality of plug-assist means interlinked so as to form an orbiting device having a plug-assist member engaging the sheet material about a substantial arc of wheel surface. The Winstead '964 patent teaches an apparatus for continuously extruding and forming molded products from a web of thermoplastic material while continuously separating the product from the web, stacking and handling the products, and recycling the web selvage for further extrusion. The apparatus uses multiple mode cavities in a rotating polygon configuration over a peripheral surface of which the biaxially oriented web is continuously positioned by a follower roller interfacing the polygon with a biaxial orientation device. The Winstead U.S. Pat. No. 3,789,095 is an integrated method of continuously extruding low density form thermoplastic material and manufacturing three-dimensional formed articles therefrom.
The Howell U.S. Pat. No. 3,868,209, is a twin sheet thermoformer for fabricating a hollow plastic object from a pair of heat-fusible thermoplastic sheets which are serially moved in a common horizontal plane from a heating station to a mold mechanism at a forming station. The Held, Jr. patent, U.S. Pat. No. 3,695,799, is an apparatus for vacuum forming hollow articles from two sheets of thermoplastic material by passing the sheets of material through a heating zone while in a spaced relationship and between two mold halves. The mold halves are brought together as a vacuum is pulled on each sheet to cause it to conform to the shape of its respective mold so as to mold a hollow article. The Budzynski et al., U.S. Pat. No. 5,551,860, is a blow molding apparatus for making bottles which have rotating molds continuously rotating while aligning one mold at a time with an extrusion die handle for loading the mold. The Hujik patent, U.S. Pat. No. 3,915,608, is an injection molding machine for multi-layered shoe soles which includes a turntable for rotating a plurality of molds through a plurality of work stations for continuously molding shoe soles. The Ludwig patent, U.S. Pat. No. 3,302,243, is another apparatus for injection molding of plastic shoes. The Lameris et al. patent, U.S. Pat. No. 3,224,043, teaches an injection molding machine having at least two molds which can be rotated for alignment with plastic injecting nozzles. The Vismara patent, U.S. Pat. No. 4,698,001, is a machine for manufacturing molded plastic motorcycle helmets and which uses a compression type mold in which a pair of mold halves is shifted between positions. The Krumm patent, U.S. Pat. No. 4,304,622, is an apparatus for producing thick slabs of thermoplastic synthetic resins which includes a pair of extruders, each extruding a half slab strand to a respective roller assembly. The roller assemblies have final rollers which form a consolidation nip between them in which the two half slabs are bonded together.
Composites and Other Processes
Composites are materials formed from a mixture of two or more components that produce a material with properties or characteristics that are superior to those of the individual materials. Most composites comprise two parts, namely a matrix component and reinforcement component(s). Matrix components are the materials that bind the composite together and they are usually less stiff than the reinforcement components. These materials are shaped under pressure at elevated temperatures. The matrix encapsulates the reinforcements in place and distributes the load among the reinforcements. Since reinforcements are usually stiffer than the matrix material, they are the primary load-carrying component within the composite. Reinforcements may come in many different forms ranging from fibers, to fabrics, to particles or rods imbedded into the matrix that form the composite.
Composite structures have existed for millions of years in nature. Examination of the microstructure of wood or the bioceramics of a seashell reveals the occurrence of composites found in nature and indicates that modem composite materials have essentially evolved to mimic structures found in nature. A perfect example of a composite material is concrete. Different forms of concrete offer an insight as to how reinforcements work. The cement acts as the matrix, which holds the elements together, while the sand, gravel, and steel, act as reinforcements. Concrete made with only sand and cement is not nearly as strong as concrete made from cement, sand, and stones, which, in turn, is not as strong as concrete reinforced with steel, sand and stones. The matrix and reinforcement materials of concrete are blended, poured and molded, typically in a form structure. In the generation of parts made with other composite materials, the shape of a composite structure or part is determined by the shape or geometry of the mold, die or other tooling used to form the composite structures.
There are many different types of composites, including plastic composites. Each plastic resin has its own unique properties, which when combined with different reinforcements create composites with different mechanical and physical properties. If one considered the number of plastic polymers in existence today and multiplied that figure by the number of reinforcements available, the number of potential composite materials is staggering. Plastic composites are classified within two primary categories: thermoset and thermoplastic composites.
In the case of thermoset composites, after application of heat and pressure, thermoset resins undergo a chemical change, which cross-links the molecular structure of the material. Once cured, a thermoset part cannot be remolded. Thermoset plastics resist higher temperatures and provide greater dimensional stability than most thermoplastics because of the tightly cross-linked structure found in thermoset plastic. Thermoplastic matrix components are not as constrained as thermoset materials and can be recycled and reshaped to create a new part.
Common matrix components for thermoplastic composites include polypropylene (PP), polyethylene (PE), polyetheretherketone (PEEK) and nylon. Thermoplastics that are reinforced with high-strength, high-modulus fibers to form thermoplastic composites provide dramatic increases in strength and stiffness, as well as toughness and dimensional stability.
Composite materials are used in numerous applications across a broad range of industries. Typically, composites are used to replace products made of metal alloys or multi-component metal structures assembled with fasteners or other connectors. Composites offer sufficient strength, while providing a reduction in weight. This is particularly important in industries such as automotive and aerospace, where the use of composite materials results in lighter, faster, more fuel-efficient and environmentally robust aircraft and automobiles. Composites may also be designed to replace wood, fiberglass and other more traditional materials. The following is a partial list of industries that may have application for the use of large parts made from thermoplastic composite materials: aerospace, automotive, construction, home appliance, marine, material handling, medical, military, telecommunications, transportation and waste management.
In general, among other attributes, thermoplastic composite materials are resistant to corrosion and offer long fatigue lives making them particularly attractive for many manufacturers. The fatigue life refers to the period of time that a part lasts prior to exhibiting material wear or significant stress, to the point of impairing the ability of the part to perform to specification. Typically, composites are utilized in applications where there is a desire to reduce the weight of a particular part while providing the strength and other desirable properties of the existing part. There are a number of parts made from thermoset composite materials that are quite expensive. These types of parts are typically referred to as advanced composite materials and are utilized most often in the military and aerospace industries.
Product development engineers and production engineers believe that thermoplastic composite materials will play an ever-increasing role in modem technological development. New thermoplastic resins are regularly developed and more innovative methods of manufacturing are being introduced to lower the costs associated with manufacturing parts made from composite materials. As the cost for manufacturing parts made with thermoplastic composite materials reduces, the use of thermoplastic composites becomes a more viable solution for many commercial and industrial applications.
Molding Methods Currently Available for Thermoplastic Composites
Most of the commercially available manufacturing technology for thermoplastic composites was adapted from methods for processing thermoset composites. Since these methods are designed for resin systems with much lower viscosities and longer cure times, certain inefficiencies and difficulties have plagued the thermoplastic manufacturing process. There are several methods of manufacturing with thermoplastic composites currently in use. Some of the most common processes include compression molding, injection molding, and autoclave processing, all of which can be used for the production of “near-net shape” parts, i.e., parts that substantially conform to the desired or designed shape after molding. Less common methods for process thermoplastic composites include pultrusion, vacuum forming, diaphragm forming and hot press techniques.
Compression Molding
Compression molding is by far the most widespread method currently used for commercially manufacturing structural thermoplastic composite components. Typically, compression molding utilizes a glass mat thermoplastic (GMT) composite comprising polypropylene or a similar matrix that is blended with continuous or chopped, randomly oriented glass fibers. GMT is produced by third-party material compounders, and sold as standard or custom size flat blanks to be molded. Using this pre-impregnated composite (or pre-preg as it is more commonly called when using its thermoset equivalent), pieces of GMT are heated in an oven, and then laid on a molding tool. The two matched halves of the molding tool are closed under great pressure, forcing the resin and fibers to fill the entire mold cavity. Once the part is cooled, it is removed from the mold with the assistance of an ejecting mechanism.
Generally, the matched molding tools used for GMT forming are machined from high strength steel to endure the continuous application of the high molding pressure without degradation. These molds are often actively heated and cooled to accelerate cycle times and improve the surface finish quality. GMT molding is considered one of the most productive composite manufacturing processes with cycle times ranging between 30 and 90 seconds. Compression molding does require a high capital investment, however, to purchase high capacity presses (2000–3000 tons of pressure) and high pressure molds, therefore it is only efficient for large production volumes. Lower volumes of smaller parts can be manufactured using aluminum molds on existing presses to save some cost. Other disadvantages of the process are low fiber fractions (20% to 30%) due to viscosity problems, and the ability to only obtain intermediate quality surface finishes.
Injection Molding
Injection molding is the most prevalent method of manufacturing for non-reinforced thermoplastic parts, and is becoming more commonly used for short-fiber reinforced thermoplastic composites. Using this method, thermoplastic pellets are impregnated with short fibers and extruded into a closed two-part hardened steel tool at injection pressures usually ranging from 15,000 to 30,000 psi. Molds are heated to achieve high flow and then cooled instantly to minimize distortion. Using fluid dynamic analysis, molds can be designed which yield fibers with specific orientations in various locations, but generically injection molded parts are isotropic. The fibers in the final parts typically are no more than one-eighth (⅛)″ long, and the maximum fiber volume content is about 40%. A slight variation of this method is known as resin transfer molding (RTM). RTM manufacturing utilizes matted fibers that are placed in a mold which is then charged with resin under high pressure. This method has the advantages of being able to manually orient fibers and use longer fiber lengths.
Injection molding is the fastest of the thermoplastic processes, and thus is generally used for large volume applications such as automotive and consumer goods. The cycle times range between 20 and 60 seconds. Injection molding also produces highly repeatable near-net shaped parts. The ability to mold around inserts, holes and core material is another advantage. Finally, injection molding and RTM generally offer the best surface finish of any process.
The process discussed above suffers from real limitations with respect to the size and weight of parts that can be produced by injection molding, because of the size of the required molds and capacity of injection molding machines. Therefore, this method has been reserved for small to medium size production parts. Most problematic from a structural reinforcing point is the limitation regarding the length of reinforcement fiber that can be used in the injection molding process.
Autoclave Processing
Autoclave processing is yet another thermoplastic composite manufacturing process used by the industry. Thermoplastic prepregs with unidirectional fibers or woven fabrics are laid over a single sided tool. Several layers of bagging material are placed over the pre-preg assembly for surface finish, to prevent sticking, and to enable a vacuum to be drawn once it is placed in an autoclave. Inside the autoclave, the composite material is heated up and put under pressure to consolidate and cross-link the layers of material. Unlike compression and injection molding, the tool is an open mold and can be made of either aluminum or steel since the pressures involved are much lower.
Because the autoclave process is much slower and more labor intensive, it is utilized primarily for very large, low volume parts that require a high degree of accuracy; it is not conducive for production lines. Significant advantages of this method include high fiber volume fractions and control of the fiber orientation for enabling specific material properties. This process is particularly useful for prototyping because the tooling is relatively inexpensive.
Molding Methods for Thermoplastic Composites Requiring “Long” Fibers
None of the processes described above are capable of producing a thermoplastic composite reinforced with long fibers (i.e., greater than about one-half inch) that remain largely unbroken during the molding process itself; this is especially true for the production of large and more complex parts. Historically, a three-step process was utilized to mold such a part: (1) third party compounding of pre-preg composite formulation; (2) preheating of pre-preg material in oven, and, (3) insertion of molten material in a mold to form a desired part. This process has several disadvantages that limit the industry's versatility for producing more complex, large parts with sufficient structural reinforcement.
One disadvantage is that the sheet-molding process cannot produce a part of varying thickness, or parts requiring “deep draw” of thermoplastic composite material. The thicker the extruded sheet, the more difficult it is to re-melt the sheet uniformly through its thickness to avoid problems associated with the structural formation of the final part. For example, a pallet having feet extruding perpendicularly from the top surface is a deep draw portion of the pallet that cannot be molded using a thicker extruded sheet because the formation of the pallet feet requires a deep draw of material in the “vertical plane” and, as such, will not be uniform over the horizontal plane of the extruded sheet. Other disadvantages associated with the geometric restrictions of an extruded sheet having a uniform thickness are apparent and will be described in more detail below in conjunction with the description of the present invention.
The present invention is directed towards a molding system for producing a thermoplastic resin of thermoplastic composite parts using either a vacuum or compression mold with parts being fed directly to the molds from an extrusion die while the thermoplastic slab still retains the heat used in heating the resins to a fluid state for forming the sheets of material through the extrusion die. The present invention relates to a thermoplastic molding process and apparatus and especially to a thermoplastic process and apparatus using a thermoplastic extrusion die having adjustable gates for varying the thickness of the extruded material, which material is molded as it is passed from the extrusion die.
The present invention is further directed towards a continual thermoforming system which is fed slabs of thermoplastic material directly from an extruder forming the slabs of material onto a mold which can be rotated between stations. The thermoplastic material is extruded through an extrusion die which is adjustable for providing deviations from a constant thickness plastic slab to a variable thickness across the surface of the plastic slab. The variable thickness can be adjusted for any particular molding run or can be continuously varied as desired. This allows for continuous molding or thermoplastic material having different thickness across the extruded slab and through the molded part to control the interim part thickness of the molded part so that the molded part can have thick or thin spots as desired throughout the molded part. The present invention is not limited as to size, shape, composition, weight or strength of a desired part manufactured by the extrusion molding process.